math/0510253Oct 12, 200532 pages, LaTeX. v2:Substantial revision, distinguishing between comodules of both constituent bialgebroids in a Hopf algebroid
The notions of a cleft extension and a cross product with a Hopf algebroid are introduced and studied. In particular it is shown that an extension (with a Hopf algebroid H=(HL,HR)) is cleft if and only if it is HR
Nearby in the stack
-Galois and has a normal basis property relative to the base ring
L
of
HL
. Cleft extensions are identified as crossed products with invertible cocycles. The relationship between the equivalence classes of crossed products and gauge transformations is established. Strong connections in cleft extensions are classified and sufficient conditions are derived for the Chern-Galois characters to be independent on the choice of strong connections. The results concerning cleft extensions and crossed product are then extended to the case of weak cleft extensions of Hopf algebroids hereby defined.